The present invention relates to a method for removing of ammonia and ammonia compounds from fly ash and other combustion byproducts.
In recent years, there has been increasing public and government concern over the environmental impact of nitrogen oxides (NO.sub.x) emissions, which contributes to the environmental impact of acid rain. Power plants and other industrial plants based on combustion processes are by far the largest producers of NO.sub.x pollution. In the United States, the Clean Air Act Amendment of 1990 called for a significant reduction in the NO.sub.x emissions from such plants.
It is known in the art that ammonia or ammonia compounds can be utilized to reduce nitrogen oxides emissions by selective catalytic or non-catalytic reduction. Ammonia or ammonia compounds react with and convert the nitrogen oxides to nitrogen. The following is an example of the reaction: NH.sub.3 +4 NO+0.sub.2.fwdarw.4 N.sub.2 +6 H.sub.2 O.
However, while ammonia has been used successfully to reduce nitrogen oxide emissions, the ammonia is typically introduced in excess for the reaction and is not totally consumed. Therefore, residues from the combustion process such as fly ash and other combustion byproducts will contain ammonia and/or ammonia compounds (such as ammonia sulfates, ammonia chlorides, ammonia hydroxide, ammonia carbonates).
Ammonia has also been utilized to help reduce particulate emissions, which are closely regulated by the government. For example, ammonia can be added as a conditioning agent in electrostatic precipitators to modify the properties of fly ash for improved collection efficiency. In this process, ammonia and ammonia compounds are also deposited on fly ash. See J. R. Turner et al in 1994,(l .R. Turner, S. Chone and M. P. Dudukovic, "Ammonia/Fly ash Interaction and Their Impact on Flue Gas Treatment Technologies," Chemical Engineering Science, V. 49,No. 24A, pp, 4315-4325, 1994.)
The deposition of ammonia and ammonia compounds on combustion by-products such as fly ash can cause problems for the utilization of the by-products. For example, about 10 million tons of fly ash are currently utilized annually in the United States for concrete applications. When ammonia-contaminated fly ash is used in such applications, hazardous levels of ammonia odor may be generated. Concentrations of ammonia as low as 100 ppm can be an odor irritant as well as an irritant to the eyes. In order to use the fly ash in concrete or other applications, the ammonia content of the fly ash should desirably be below 60-80 ppm (0.060-0.080 mg NH.sub.3 /g fly ash). Accordingly, it is important to reduce or remove ammonia and ammonia compounds from combustion by-products such as fly ash prior to their utilization in other applications.
U.S. Pat. No. 4,288,420 teaches a method for the removal of nitrogen oxides and ash from a coal-fired boiler by treatment with ammonia in which the ammonia is separated from the coal ash by heating the exhaust gas at a temperature of 350 to 700.degree. C. U.S. Pat. No. 4,911,900 describes the combination of various ammonia stripping and scrubbing processes to recover ammonia for nitrogen oxide reduction. However, these methods are focused on the recovery of ammonia for reuse in nitrogen oxides reduction, and do not provide a method for the removal of residual ammonia on fly ash which is to be used in other applications.
Many processes have been developed for the beneficiation of fly ash. For example, U.S. Pat. Nos. 5,227,047 and 5,456,363 describe wet processes including froth flotation, magnetic separation and others to separate various fly ash components, such as carbon, aluminum silicates, cenospheres, and iron oxides. However, these processes do not consider the effects of ammonia on the separated products. For example, when the ammonia contaminated fly ash is subjected to such beneficiation processes, ammonia and its compounds are dissolved in the process water. If the separated products are utilized while containing a portion of the process water, ammonia contaminants may still be present in the separated products, causing possible problems for their utilization. In addition, the process water is frequently filtered from the separated products and recirculated. Thus, the ammonia concentration in the process water can build up to very high concentrations.
Accordingly, there is still a need in the art for a method of reducing or removing ammonia and/or ammonia compounds from combustion by-products such as fly ash prior to their utilization.